Anti-alumina-buildup refractories for casting nozzles

ABSTRACT

Disclosed is anti-alumina-buildup refractories for casting nozzles, which comprises a refractory aggregate including 20 mass % or more of CaO component, and 10 mass % or more of clinker particles each containing CaO as a mineral phase, on the basis of 100 mass % of the entire composition. At least a part of the surfaces of the CaO exposed from the surfaces of the corresponding clinker particles is formed with a CaCO 3  film. The CaCO 3  film releases CO 2  gas through thermal decomposition to smooth an operative surface of the nozzle so as to prevent the accretion of metal thereon, so that CaO is continuously supplied to alumina attached on the operative surface to prevent alumina buildup. In addition, the CaCO 3  film effectively prevents the hydration of CaO due to a hydration reaction.

TECHNICAL FIELD

The present invention relates to refractories applicable to a nozzle forcontinuous casting of steel, such as immersion nozzle, upper nozzle,sliding nozzle, lower nozzle or long nozzle, and more particularly torefractories applicable to an inner hole portion of such a castingnozzle to be in contact with molten steel, to prevent alumina buildupthereon.

BACKGROUND ART

Responding to recent requirements stricter quality control of steelproducts, a good deal of effort has been made to reduce non-metallicinclusions, such as alumina which builds up on an inner hole portion ofa nozzle for continuous casting of steel. Moreover, in terms of castingoperations, the buildup of alumina and others which arises from longhours of casting is likely to cause clogging of the inner hole portionand difficulties in continuing the casting operation, which hinders theimprovement in productivity.

As one of measures against the alumina buildup, there has been known atechnique in which argon gas is injected from the inner surface of anozzle into molten steel to prevent the alumina buildup in physicalmanner. In this technique, if the argon gas is injected at an excessiveamount, bubbles of the injected argon gas will be incorporated intomolten steel to form pinholes in slabs. Thus, due to the restriction inthe allowable injection amount of argon gas, this method cannot beexactly used as a sufficient measure for preventing the alumina buildup.

There has also been known a technique intended to provide ananti-alumina-buildup function to refractories themselves. For example,in view of inducing the reaction between alumina attached on arefractory nozzle and CaO contained in the refractories to form alow-melting-point or fusible substance, Japanese Patent Publication No.61-44836 discloses a casting nozzle using refractories containing acombination of graphite, and either one of sintered calcia, fused calciaand another ceramic material containing a CaO component. While suchCaO-containing refractories can exhibit an anti-alumina-buildup effectin some cases, it is likely to cause increase in alumina buildupdepending on the circumstances.

Further, when applying refractories containing CaO as a mineral phase toa casting nozzle, the CaO is likely to undesirably hydrated due to ahydration reaction with water absorbed therein. While Japanese PatentLaid-Open Publication No. 10-5944 discloses a technique of adding CaCO₃as aggregate to prevent the hydration, the CaCO₃ aggregate ispyrolytically decomposed to have a high porosity, and the resultingirregularity of an effective or operative surface of a casting nozzleleads to undesirable alumina buildup thereon.

DISCLOSURE OF INVENTION

In view of the above problems concerning the measures against aluminumbuildup on CaO-containing refractory nozzles, it is therefor an objectof the present invention to provide refractories for casting nozzlescapable of bringing out an excellent effect of suppressing the aluminabuildup on an operative surface, and an effect of preventing thehydration of CaO due to a hydration reaction.

Through various research on achieving this object, based on analyses ofthe phenomenon of alumina attachment/buildup on an operative surface ofCaO-containing refractories, it was found that metal is first attachedon the surface of the refractories, and alumina is then attached on themetal.

According to the observation of a portion of the surface having themetal attached thereon, the portion of the refractory surface had alarge irregularity. Based on this fact, it is verified that thisirregularity generates stagnation in molten flow to accelerate theaccretion of metal thereon, and the accreted metal blocks the dispersionof CaO from refractories constituting a casting nozzle (hereinafteroccasionally referred to as “nozzle”) to preclude alumina attached onthe metal from being converted to a fusible substance.

Using the above knowledge that the alumina buildup is caused by thepenetration of metal, such as molten steel, into the refractory matrixin advance of alumina, the inventor found that the penetration of moltensteel into the matrix of refractories constituting a nozzle andincluding clinker particles each including CaO as a mineral phase can besuppressed by forming CaCO₃ on at least a part of the surfaces of theCaO exposed outside from the surfaces of the corresponding clinkerparticles, so as to achieve an enhanced anti-alumina-buildup function,and finally accomplish the present invention. While it is preferable toform CaCO₃ on all of the surfaces of the CaO exposed outside from thesurfaces of the corresponding clinker particles, the CaCO₃ may be formedon only a part of the surfaces of the CaO to obtain theanti-alumina-buildup accordingly.

Specifically, the present invention provides refractories applicable toa casting nozzle, primarily an inner hole portion of the nozzle, whichcomprises refractory aggregate including 20 mass % or more of CaOcomponent, and 10 mass % or more of clinker particles each containingCaO as a mineral phase, on the basis of 100 mass % of the entirecomposition. In these refractories, at least a part of the surfaces ofthe CaO exposed from the surfaces of the corresponding clinker particlesis formed with a CaCO₃ film.

A function to be contained from the formation of CaCO₃ on at least apart of the exposed surfaces of the CaO will be described below based onreaction formulas.

CaCO₃ on the surface of the clinker is decomposed by a heat load frommolten steel to release CO₂ gas.

CaCO₃→CaO+CO₂

The released CO₂ reacts with aluminum in the molten steel to formalumina on the operative surface of the nozzle.

4Al+3CO₂→2Al₂O₃+3C

The formed alumina reacts with CaO to form a fusible substance so as tosmooth the operative surface. This prevents the accretion of metal whilefacilitating the continuous supply of CaO to alumina attached on theoperative surface, to suppress/prevent the alumina buildup.

As above, the suppression of the alumina buildup can be achieved only ifCaCO₃ is allowed to exist only on the surface of the clinker particle soas to maintain the smoothness of the operative surface. When using CaCO₃as aggregate as in the aforementioned Japanese Patent Laid-OpenPublication No. 10-5944, the smoothness of an operative surface cannotbe maintained because the CaCO₃ aggregate itself has a high porosity.

The formation of CaCO₃ on the surface of CaO exposed outside from thesurface of the clinker particle in the refractories additionallyprovides a function of preventing the hydration of CaO due to ahydration reaction. To facilitate the reaction between aluminaprecipitated in molten steel and the CaO in the nozzle itself, to form afusible substance, the CaO may be contained in the refractories at 20mass % or more on the basis of 100 mass % of the entire nozzlecomposition.

In the present invention, the aggregate may include 20 mass % or more ofCaO component, and 10 mass % or more of clinker particles eachcontaining CaO as a mineral phase, on the basis of 100 mass % of theentire composition. The aggregate may be used together with anotherrefractory aggregate without any problem.

In view of the anti-alumina-buildup effect, a clinker particlecontaining CaO as a mineral phase, so-called free-CaO, at 10 mass % ormore, preferably 20 mass % or more, more preferably 30 mass % or more,specifically calcia-magnesia clinker including calcia clinker anddolomite clinker, may be preferably used to facilitate the formation ofthe CaCO₃ film on the surface of CaO in the clinker particle.

Any combination of CaZrO₃ clinker or CaO-SiO clinker, which contains noCaO as a mineral phase; material containing CaO-based compound as aprimary component; carbon-based material, such as graphite or carbonblack; and magnesia-based or zirconia-based material commonly used withrefractories, may be additionally used as aggregate according to need.

The following technique may be used to form the CaCO₃ film on at least apart of the surfaces of the CaO exposed outside from the surfaces of thecorresponding clinker particles.

(1) A nozzle molded integrally with the refractories is subjected to aheat treatment under a CO₂ atmosphere at a temperature of 300 to 850° C.Alternatively, the molded nozzle is subjected to a heat treatment at atemperature of greater than 850° C., and then cooled under a CO₂atmosphere in the temperature range of 300 to 850° C.

(2) The molded nozzle is burnt in a conventional process, and is thensubjected to a heat treatment under a CO₂ atmosphere at a temperature of300 to 850° C., again.

CaCO₃ may be formed on the surface of the clinker particle to have athickness in the range of 0.07 to 7 μm, preferably 0.1 to 5 μm, morepreferably 0.5 to 5 μm. If the thickness is less than 0.1 μm, the amountof CO₂ to be generated from thermal decomposition will be reduced, andconsequently the amount of alumina to be formed on the operative surfacewill be reduced. While the thickness of less than 0.1 μm causesdeterioration in the anti-hydration effect, if the thickness is at least0.07 μm, a practically acceptable level can be maintained because thealumina buildup will occur only in a significantly limited area, and thedeterioration in the anti-hydration effect is not significant unless thenozzle is left in nonuse for a long period of time. If the thickness isgreater than 5 μm, the surface of the clinker will have an increasedporosity after the release of CO₂ likely to case difficulty in obtainingsmoothness in the operative surface. However, if the thickness is 7 μmor less, a practically acceptable level can be remained because thealumina buildup caused by the increase porosity in the operative surfaceoccurs only in a partial area. In this case, the anti-hydration effectis not particularly deteriorated. The thickness of the CaCO₃ film can becontrolled by adjusting at least one of the CO₂ concentration, time andtemperature for inducing the reaction between CaO and CO₂.

CaCO₃ is thermally decomposed at a temperature of about 900° C. or more.Thus, where a preheat temperature of the nozzle is set at 900° C. ormore, the release of CO₂ before the inflow of molten steel is likely tospoil the anti-alumina-buildup effect. As one measure against thisproblem, an antioxidant may be applied on the CaCO₃ film. Specifically,this antioxidant is fused to form a glass layer on the CaCO₃ film, andthe glass layer acts to suppress the release of CO₂ until the inflow ofmolten steel, so as to allow the anti-alumina-buildup effect to beadequately brought out.

Preferably, the refractories of the present invention primarily designedto obtain the anti-alumina-buildup effect are applied to a surface ofcontinuous casting refractories to be in contact with molten steel. Inthis case, the refractory use as an inner hole portion may be integrallymolded together with other nozzle component materials, or may be formedas sleeve-shaped refractories and inserted into or adhesively bonded toa refractory nozzle body.

In the present invention, the CaCO₃ film is formed only on the surfaceof CaO exposed outside from the surface of the clinker particle. Thus,the increase in porosity due to decomposition of CaCO₃ can be minimizedto achieve approximately no deterioration in strength afterdecomposition of CaCO₃.

Thus, the refractories of the present invention are applicable to notonly an inner hole portion but also an nozzle body. In addition, nozzlebody and the inner hole portion may be formed using the same material toprovide enhanced productivity. The refractories of the present inventionmay also be applied as a baffle for preventing uneven molten steel flowin the nozzle.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention will now be described inconnection of Examples.

Table 1 shows the composition of a compound for use in formingrefractories of the present invention, the conditions of a treatment forforming CaCO₃, and characteristics of obtained refractories, togetherwith Comparative Example.

TABLE 1 Comparative Example Inventive Example 1 1 2 3 4 5 6 7compounding graphite (0.5 mm or less) 30 30 30 30 30 30 30 30 ratiodolomite clinker (1 mm or less) 20 20 20 20 20 20 20 20 (%) dolomiteclinker (0.2 mm or less) 30 30 30 30 30 30 30 30 dolomite clinker (0.074mm or less) 20 20 20 20 20 20 20 20 Treatment Under CO₂ Non Yes Yes YesYes Yes Yes Yes atmosphere during cooling measured CaCO3 film thickness(μm) 0 0.07 0.1 0.5 1 2 5 7 value alumina buildup test* X Δ ◯ ◯ ◯ ◯ ◯ Δhydration test (index of strength) 7 73 94 98 100 100 100 100 *Aluminabuildup test: ◯: no buildup, Δ: buildup only in a part of the surface,X: buildup in 50% or more of the entire surface

An appropriate amount of phenol resin as organic binder was added toeach of the materials as shown in Table 1, and the mixture washomogenously kneaded. The obtained compound was subjected to a coldisostatic pressing (CIP) process under a pressure of 1000 Kg/cm², andburnt under a reduction atmosphere at a temperature of 1000° C.

In Comparative Example, the burnt product was directly cooled. InInventive Examples 1 to 7, when a temperature in a furnace was reducedto 850° C. during the course of the cooling, CO₂ was supplied into thefurnace to form a CaCO₃ film on the surfaces of CaO exposed outside fromthe surfaces of respective dolomite clinker particles.

The thickness of the CaCO₃ film was measured by observing the cutsection of the clinker particle using a scanning electron microscope,and controlled by adjusting the partial pressure and supply time of CO₂.

In the alumina buildup test, molten steel of low-carbon aluminum-killedsteel with 0.2% of aluminum dissolved therein was maintained at 1570°C., and a rod-shaped sample was immersed into the molten steel. After 60minutes, the sample was pulled out to evaluate the amount of aluminabuildup. In Table 1, ◯ indicates a sample having no alumina buildup, Δindicated a sample having alumina buildup only in a part of the surface,and x indicates a sample having alumina buildup in 50% or more of theentire surface.

The hydration test was performed by measuring the bending strength of asample after being maintained under a temperature of 35° C. and arelative humidity of 70% for 3 days, and expressing the measured bendingstrength value as an index on the basis of 100 of the bending strengthof the sample before the test. An index closer to 100 indicates a betterresult or less deterioration in the strength.

As compared to Comparative Example, all of Inventive Examples 1 to 7have better results in both the alumina buildup test and the hydrationtest. The anti-alumina-buildup and anti-hydration effects from CaCO₃were observed in Inventive Examples 1 to 7 having a film thicknesses of0.07 μm to 7 μm. An optimal film thickness was in the range of 0.1 to 5μm as in Inventive Examples 2 to 6. Among them, Inventive Examples 3 to6 having a film thickness of 0.5 to 5 μm had particularly good results.

As mentioned above, in the continuous casting refractories includingclinker particles each containing CaO as a mineral phase, the CaCO₃ filmformed on the surfaces of the CaO exposed outside from the surfaces ofthe respective clinker particles releases CO₂ gas through thermaldecomposition to smooth the operative surface so as to prevent theaccretion of metal thereon, so that CaO is continuously supplied toalumina attached on the operative surface to prevent the aluminabuildup. In addition, the CaCO₃ film effectively prevents the hydrationof CaO due to a hydration reaction so as to provide enhanced operationalefficiency and steel quality in continuous casting.

INDUSTRIAL APPLICABILITY

The present invention is applicable as anti-alumina-buildup refractoriesfor use in inner hole portions of various nozzles for continuous castingof steel.

1. Anti-alumina-buildup refractories for casting nozzles, comprising arefractory aggregate which includes 20 mass % or more of CaO component,and 10 mass % or more of clinker particles on the basis of 100 mass % ofthe entire composition, each of said clinker particles containing CaO asa mineral phase, wherein at least a part of the surfaces of the CaOexposed from the surfaces of said corresponding clinker particles isformed with a CaCO₃ film.
 2. The anti-alumina-buildup refractories asdefined in claim 1, wherein said CaCO₃ film has a thickness ranging from0.1 to 5 μm.